I don't think
@iserith ever responded to this. I was going to leave it to them, because I learned my solution from them. Which is to allow pvp, but the outcome of every adversarial reaction is narrated, without a roll, not by the DM but by the
target.
So when you stab Bob in the eye, the DM turns to Bob and says, "What happens?" Bob might say, "The dagger glances off my helmet. I cast command and tell him to grovel." So the DM turns to you and says, "What happens?" And you say, "I know Bob's tricks so the spell doesn't work on me; no saving throw needed."
Etc.
I.e., the "soft veto".